In the erection of above-ground structures it is often necessary to form inground retaining walls for use as a load bearing foundation or about a site to be excavated, as a means of retaining the earth about the excavation. Where such excavations are adjacent an existing structure the retaining wall or walls along the excavation line adjacent the existing building must be formed to resist soil pressures established beneath the adjoining structure. In particular, such retaining walls must be put in place so that the soil beneath the adjoining structure will not collapse outwardly into the excavation as the latter progresses. Walls of this type may also be constructed to cut off ground water movement.
Various methods have been employed to form such walls including driving sheet piling along the excavation line to form a retaining wall, and boring a closely spaced row of holes and casting reinforced concrete piles in the respective holes to form the desired wall. If the latter process is used for retaining walls, it is necessary to add substantial horizontal strengthening to the row of piles to maintain their alignment. It is also difficult to form such walls as watertight cutoff walls. Continuous reinforced concrete walls provided excellent supporting, retaining and/or cutoff walls. However to date, it is generally not possible to form inground reinforced concrete walls economically.
Techniques for the continuous excavation of trenches of cross-section suitable for such retaining and/or cutoff walls are available. A typical machine is the subject of Australian patent application No. 41139/85. Such machines can continuously excavate a relatively deep trench along an excavation line. However, their trenching capacity is limited by flexural and machine weight considerations. Accordingly, while it may be possible to construct trenching apparatus which would excavate any desired trench, the size of the machine necessary for normal inground retaining wall trenching operations would render it impractical.
Furthermore, such trenching machines may incorporate an arm supporting an endless chain carrying cutting teeth for their trenching tools. The teeth generally project a significant distance outward from the pivots of the chain, so that cutting forces acting at a cutting tooth in a direction along the line of the chain impose bending moments which tend to flex the cutting chain in such a way as to increase the cutting depth of the cutting tooth uncontrollably, resulting in excessive loading on the cutting tooth, the chain and the trenching machine. The problem could be alleviated by using a chain of much longer pitch, but this approach would require the use iof correspondingly larger chain sprockets.
Such trenching arms must be forced forward in the excavation by advancing the machine to which the cutting arm is attached. As the depth of excavation becomes greater, this technique places large bending moments on the cutting arm and its support structure, due to the increasing vertical offset between the centroid of the normal forces on the cutting teeth and the line of action of the advance force at ground level. This problem can be alleviated by using an advance system placed within the trenching arm below ground level to force the trenching arm forward by engaging plates with the sides of the excavation and forcing the trenching arm forward by pushing the plates rearward relative to the arm.